As is known, most of the perspiration of the foot originates at the interface between the sole of the foot and the sole of the shoe, and the perspiration that forms there, being unable to evaporate, condenses on the plantar insert on which the foot rests; only a minimal part of the perspiration evaporates through the upper.
This phenomenon is particularly conspicuous in shoes which have a rubber sole; in these cases, vapor permeation through the sole is prevented completely.
In order to solve this problem, shoes with soles which are waterproof and vapor-permeable (permeable to water vapor) and are made of plastic material have now been devised for several years.
Their purpose is to allow the escape of the water vapor produced by the foot with perspiration.
One of the solutions is disclosed for example in U.S. Pat. No. 5,044,096 and EP 382904 and consists in dividing the rubber sole into two layers provided with through holes (the tread is associated with the lower layer) and in interposing a membrane which is waterproof and vapor-permeable and is connected perimetrically and hermetically to the two layers so as to not allow water infiltrations.
It is convenient to arrange below the membrane a layer for protecting it (for example a felt layer), as disclosed for example in U.S. Pat. No. 5,983,524 and EP 858270.
The need to prevent foreign objects of a certain size from being able to arrange themselves inside the holes, with the possibility of damaging both the felt and the membrane, forces the use of holes having a size of 1.5-2.0 mm, which are spaced enough to avoid compromising the structural strength of the tread, thus reducing the vapor permeation area.
One optimum solution meant to increase the vapor permeation area of the tread is disclosed for example in U.S. Ser. No. 10/527,187 and in EPA 03769286.0.
The type of sole disclosed in this last patent application is particularly adapted to dissipate the large amounts of water vapor that form, with perspiration, inside the shoes of individuals who have higher-than-average perspiration.
Excessive foot perspiration can also occur in the case of extremely hot and humid climates and if shoes are used for sports activities.
The cited patent application discloses a sole which has a structure comprising a supporting layer which, at least in a preset large portion, is provided by means of a net (made of synthetic or metallic material).
A membrane made of a material which is impermeable to water and permeable to water vapor is associated in an upward region with the supporting layer at least in the provided large net portion, which it covers.
A plastic tread with at least one large through hole at the provided large portion is joined hermetically to the membrane and to the supporting layer at least at the perimeter of the large net portion; typically, the tread is overmolded on the net.
This solution, as mentioned, allows to utilize at best the characteristics of the waterproof and vapor-permeable membrane, but in certain applications a sole which has a large net may be too rigid or may have a limited capacity to absorb impacts with the ground.